It is common knowledge that signals can be imposed upon laser light and LEDs by various means of modulation. It is also common knowledge that the atmosphere does not possess a uniform transmittance. Two classes of phenomenon primarily cause this. The first relates to the general chemical composition of the atmosphere and consists of bands of alternating high and low transmittance as a function of carrier frequency. These are well understood and generally well behaved. Thus, one can design a class of communication lasers and/or LEDs that will take advantage of higher transmittance within a particular frequency band as opposed to adjacent bands and expect the resulting system to perform significantly better than a system that does not do so. The second phenomenon is unfortunately not as well behaved and deals with the fact that the atmosphere is not homogeneous and constant in space and in time. Due to environmental conditions such as temperature gradients, moisture content, pollutants, wind and turbulence, the atmosphere's transmittance and index of refraction changes in localized spaces. These conditions are cumulative with distance, time varying, and are generally not predictable. Therefore they are best thought of in terms of random variables and consequently systems that do not compensate for them suffer in performance and quality. The result is that the channel, though it is pure line-of-sight, is an extremely hostile channel and optical systems (i.e. laser-and/or LED based) typically are only used for short distances where the cumulative atmospheric uncertainty is small. These systems typically can only establish reliable communication channels at data levels and ranges well below what might be expected for pure line-of-sight conditions. Typical means used in an attempt to overcome these obstacles include significantly increasing the output power of the transmitter laser or LED by either pure transmit power and also by the inclusion of expensive and physically large optics in the receiver configurations. While improving performance somewhat, these measures also have substantial negative impact. Higher power lasers are substantially higher in cost and also possess a health hazard to individuals in close proximity, either willingly, or unwillingly. Additionally, these higher power units also may possess a negative environmental impact Further, the cost of optical systems can go as the square of the size of the lenses employed, so it is easy to see the impact of a hostile channel on both cost and performance.
Even though some more recent systems have attempted to use other techniques such as some spatial diversity, positive results have been limited and the requirement to still use the items discussed above is not much alleviated.